In spaces which are not ventilated or only inadequately ventilated, the concentration of carbon dioxide (CO2) increases very heavily when people are present, since it is breathed out with the breathing air. While the CO2 concentration of the ambient air is around 400 ppm, the concentration in an enclosed space may rise to 2000 ppm or more within a few minutes. Above 3000 ppm the air is felt to be unpleasant, and on occasion a slight head pressure sets in, which may lead to headache if the CO2 concentration rises further. The concentrating ability also decreases with rising CO2 concentration.
The smaller the space in which one or more persons are present, the more quickly a rise in CO2 occurs having the mentioned disadvantages. One example of such small, enclosed spaces is the passenger compartment of a motor vehicle, i.e., the interior of the vehicle, such as a car. For example, if four or five people are present in the vehicle, a substantial amount of CO2 will be liberated inside the small volume by the breathing air. Of course, it is possible to counteract the rise in CO2 by supplying fresh air, e.g., by opening a window or by supplying fresh air from outside the vehicle through a ventilation device or air conditioning system. However, this is not always desirable or expedient. On the one hand, the heat or cold present in the space is lost in this way, i.e., energy has to be used once more to heat or cool the air. Furthermore, especially in the case of motor vehicles due to the surrounding conditions, it is sometimes not possible to supply fresh air, for example if this would bring pollutants from the surrounding air that are produced by other road users into the passenger compartment, or in the case of rain or snowfall or strong wind, in which cases a window cannot be opened. For this reason, a vehicle ventilation device will be operated in such cases in the so-called recirculation mode, i.e., the interior air is circulated and held at constant temperature, for example by an air conditioning system, while no fresh air or only an extremely slight amount of fresh air is supplied in this recirculation mode. Thus, no air exchange is possible here, which is associated with a corresponding rise in CO2.
From U.S. Pat. No. 3,906,945 A it is known how to reduce CO2 contained in the air by adsorption, for which the gas containing CO2 is conveyed through a chamber containing lithium hydroxide, with which the CO2 reacts in the presence of water to form lithium hydrogen carbonate. The cleaned air can again be supplied to the space.
DE 10 2012 207 382 A1 discloses a motor vehicle in which separating agents are provided for the separation of gaseous carbon dioxide from the air of the passenger compartment. In this case, air from the passenger compartment is supplied via air delivery means to the separating agents. The separating agents are suited to generating a first air stream with a lesser CO2 concentration than the air of the passenger compartment and a second air stream, wherein the first air stream is delivered to the passenger compartment and the second air stream to the air surrounding the passenger compartment.
Another air conditioning device of this kind for an enclosed space in which a filter is operated in absorption and desorption phases is known from DE 10 2014 015 231 A1.